1. Field of the Invention
The present invention relates to an electrical transformer and more particularly to a iron core electrical transformer with one or more aluminum windings that is constructed in a manner to overcome known problems associated with aluminum wire due to oxidation.
2. Description of the Prior Art
Various transformer configurations are known in the art. With the exception of primary less donut type current transformers, most transformers include at least one primary winding and at least one secondary winding. For transformers with relatively low power ratings, for example, 5 kVa and below, the windings are known to be wound around a bobbin or a paper tube. The windings are magnetically coupled together by a principle known as mutual inductance by an iron core. The iron core is normally formed by a plurality of laminated steel (or other ferromagnetic material) plates disposed in the core of the bobbin as well as around the exterior of the windings.
Other than a special type of transformer, known as an auto-transformer, the primary and secondary windings are not connected together. Rather, the windings are connected to separate sets of primary and secondary terminals. In particular, for a single phase transformer, the ends of the electrical conductors forming the primary and secondary windings are terminated to primary and secondary terminals, respectively. The primary and secondary terminals enable the transformer to be connected to an external primary circuit and an external secondary circuit.
The ratio of the number of turns in the primary winding to the number of turns in the secondary winding, known as the turns ratio, defines the relationship between the primary voltage and current to the secondary voltage and current. In order to provide more flexibility for the transformer and to reduce inventory, many known transformers are known to be provided with one or more primary and/or secondary taps. These taps represent intermediate locations between the ends of the windings having turns less than the number of turns in the full primary or secondary windings so that the transformer can be used for applications requiring different turns ratios. It is also known to provide a primary and/or secondary tap to provide intermediate primary or secondary voltages, i.e. voltages less than the full winding voltage. These taps are also terminated to terminals to enable the taps to be connected to external circuits.
Most known transformers utilize copper conductors for both the primary and secondary windings. As is known in the art, copper has many advantages over other electrical conductors, such as aluminum. For example, copper is a better electrical conductor. In particular, by volume, the electrical conductivity of aluminum is known to be about 62% that of copper. Thus, relatively smaller copper conductors are required for a given electrical current than aluminum conductors for the same magnitude of current. Copper conductors are also known to have superior mechanical strength than aluminum conductors. Thus, in applications where relatively large short circuit currents are possible, copper conductors offer the mechanical strength to withstand the forces generated by relatively large short circuit currents.
One of the most important characteristics of copper which makes it ideal for use as an electrical conductor relates to its connectivity. In particular, connectivity relates to the connections between the electrical conductor and other devices, such as terminals to which the electrical conductor is terminated. More particularly, it is important that connections to the electrical conductor be as secure as possible. Loose connections to the electrical conductor can cause local heating which can result in a fire hazard.
Copper is essentially corrosion free. As such, connections to copper conductors are stable and can be expected to remain stable for long periods of time. Aluminum conductors, on the other hand, are subject to oxidation over time, and as such build up a film on the surface of the electrical conductor which tends to weaken connections over time. Because of this problem, aluminum is used sparingly as an electrical conductor.
One major drawback of copper is the cost. The cost of copper far exceeds the cost of aluminum especially at the present time. As such, in order to reduce the cost of certain electrical devices, such as transformers, aluminum wire is known to be used for coil windings. Examples of transformers with one or more aluminum windings are disclosed in U.S. Pat. Nos. 4,639,705 and 7,034,648, hereby incorporated by reference. Even though the transformers disclosed in those patents are relatively less expensive to manufacture than corresponding transformers with all copper windings, the aluminum wound transformers still suffer from the connectivity problem discussed above. Thus, there is a need for relatively less expensive transformers which do not have the connectivity problem discussed above.